The present invention relates to a magnetic position measurement system with field containment means. The concept of using transmitting and receiving components with electromagnetic coupling is well known with respect to biomechanics and medical diagnostics, wherein a sensor assembly is mounted on a point of interest and the position of the point is determined relative to a fixed transmitter. This information is then used by computing systems to precisely show the relative motions of the points in question, which, in the medical sense, allows instruments to be precisely located in a human body with respect to the body and each other. This allows new, advanced methods of surgery and diagnostics to be performed.
When conductive materials are present, which is often the case on, below, or near an operating table, they generate eddy current fields, which distort the received magnetic field waveform, which distorts the output of the system unless the system utilizes some distortion reducing or compensating technique. When permeable materials are present, they bend and otherwise distort the magnetic field, with effects similar to conductive materials. In a surgical theater, both conductive and permeable materials are present in substantial quantities. They are a major component of many operating tables, surrounding equipment such as carts and equipment, and are present in the movable spot lamps used to illuminate the surgical field. Many operating tables have many degrees of positional and angular freedom to allow optimal placement of the surgical field relative to the surgeon, and are designed to be extremely stable and sturdy while supporting a heavy human body. As a result of these requirements, the tables contain numerous mechanisms allowing fore, aft, up, down, sideways, roll, and tilt motions. These mechanisms are physically robust and typically fabricated from steel, so that they have substantial field distortion characteristics. Shapes may include screws, rack and pinion gears, or scissors type actuators. The table surface may be one piece, or may be divided into several sections, with each section capable of motion relative to the other sections, to allow a body to be flexed such that various stresses and relative anatomical positions are optimal for a particular surgical or diagnostic procedure. The installed bases of operating tables are extremely diverse in design, and as the tables are often in service for many decades, there are many vendors, with each vendor carrying a number of different operating table designs. This poses a significant problem for magnetic position tracking systems which are used in a critical surgical environment. The operating volume for the tracker is typically within the body which lies on top of the table. This means that the tracking system is operating in close proximity to the metallic structures on, under, and around the table. The magnetic fields are distorted by these structures, which may result in large errors in the reported magnetic sensor position. The large diversity in table designs makes it impossible to predict the severity of distortion experienced on a given table. This is an unacceptable condition for a surgical environment. Attempts to compensate for these degrading effects have been made with varying degrees of effectiveness.
One method already employed is to map the entire operating volume each time the system is used. This is very time consuming and expensive, as potentially thousands of points must be taken in a precise manner if the distortion is severe and the operating volume large. It is also unreliable since during a surgical or diagnostic procedure, the table geometry is often changed which changes the relation of the table metallic structures relative to the tracking system, thereby requiring a new map if errors cannot be tolerated. Instruments and diagnostic equipment are also introduced and removed from the vicinity of the tracking system, thus rendering a map ineffective. Also, for severe distortion, a map may become totally ineffective, as the system may, at two different physical sensor spatial points, determine the sensor to be at the same position. In this case, the output data is of minimal use.
Another known method commonly described in prior art is to use AC fields over a conductive ground plane. The ground plane attenuates the magnetic field below the plane to nearly zero, which has the benefit of making the system insensitive to metallic objects below the plane. In the case of a dipole transmitter, the "method of images" is used to compute the theoretical magnetic field vectors over the plane, which are then used to provide sensor position. This method has drawbacks. One is that near the ground plane, the magnetic field intensity is nearly zero, and the vector crossing angles are degraded, which seriously reduces system performance with respect to accuracy and noise. The net result is that the sensor must be kept a few inches above the plane. Also, the dipole must be located some distance from the ground plane in order to reduce signal losses and degraded vector crossing angles within the operating volume. For a 1 cubic foot volume, the bottom of the transmitter must be about 2 inches above the plane for acceptable performance. To compute the height at which a patient must be elevated if lying on the transmitter, the thickness of the transmitter must be added to this 2 inch figure. Transmitter size is determined by required signal level within the operating volume. Sensor coil size for minimally invasive surgical applications is about 1 mm.times.5 mm in cross-section, which is very small. The requirement for precise, low noise operation at the extreme edges of the volume requires that a relatively large magnetic field magnitude be present in order to induce sufficient signal in the small coils. Transmitter size is largely dictated by how much field it must output. Since the transmitter is typically a cube, to obtain sufficient signal within a 1 cubic foot volume with a small receiver coil, the practical transmitter dimensions are on the order of 2 inches per side. We can now see that the effective transmitter assembly in this prior art teaching, including the ground plane, is 4 inches thick. In a surgical environment, the patient must be elevated to levels which a surgeon may find uncomfortable. In addition, extra padding may become necessary if the patient must lie flat on the table. Both the transmitter and the padding must be secured to the table. In short, the configuration is cumbersome and may not allow the patient to be positioned in an optimal manner.
Placing the transmitter above the operating volume is not desirable as it will potentially interfere with the surgical field. Also, as the transmitter is placed further from the ground plane, and if the dimensions of the ground plane are fixed to be a square of about 18 inches on a side, the ground plane becomes ineffective at reducing the effects of metallic objects near the operating volume. The metal housings of the surgical lighting equipment will have a greater distorting effect in the upper portions of the operating volume, as they are closer to both the transmitter and receiver. Equipment used during the procedure, including the operating table, will cause potentially life threatening distortion, which is an unacceptable condition.
Position determination depends on relative vector magnitudes from the x, y, and z coils. Distortion effects may again be removed by using a process such as mapping. As the magnitudes of the transmitted magnetic vectors from the x, y, and z coils become more similar, a given fixed amount of error in their determination will result in an increased error in position output. Again, considering the limiting case, if the magnitudes become equal then position determination is not possible. This combined effect of reduced angle of transmitted vector intersection and reduced difference in transmitted vector magnitudes is known to those skilled in the art as vector dilution. Use of a conductive ground plane under the transmitter will cause vector dilution. The severity of the vector dilution is increased as the transmitter becomes closer to the ground plane, and is also increased as the receiver becomes further from the transmitter. Vector dilution generally imposes a practical limit on how close the transmitter of a magnetic tracking system may be placed to a conductive ground plane. For a 1 cubic foot motion box, vector dilution approaches unacceptable levels if the transmitter is placed closer than 2 inches from an infinite extent conductive ground plane. Vector dilution is also present in non-dipole transmitter configurations, and the effects of its presence are similar.
The Following Prior Art is Known to Applicant
U.S. Pat. No. 4,849,692 to Blood discloses a method of eliminating eddy current distortion effects, which are generated by conductive objects, such as the stainless steel table surface, and in other objects having large surface areas. The distortion effects of permeable metals are not addressed by this system. This means that steel structures in, around, and under the operating region of the system will distort the received magnetic fields and degrade system performance. In addition, large, thick sheets of conductive metals such as Aluminum have eddy current decay times which can exceed 200 milliseconds. If the system uses 3 time division multiplexed transmit axes plus one period where all axes are off in order to compensate for the earth's field, as described in the preferred embodiment, this means that the update rate is 1/4*(200 mS)=1.25 Hz. This is unacceptably slow for many applications.
U.S. Pat. No. 5,767,669 to Hansen, et al. describes methods for eddy current field compensation without the need to compensate for the Earth's field effects. This system has no provision for reducing the effect of nearby permeable metals, nor does it address the drawback of requiring a slow update rate while operating near large, thick sheets of highly conductive metals.
U.S. Pat. No. 5,600,330 to Blood discloses a non-dipole loop transmitter-based magnetic tracking system. This system shows reduced sensitivity to small metallic objects in the operating volume, as the field from the smaller object will fall off as 1/r 3 with r being the received distance from that object, while the field from the larger transmitting loops will fall off as 1/r 2, which yields a reduced effect from the small metallic object. Large sheets of metal, however, can have an effective loop area larger than the magnetic transmitter loops, which diminishes this advantage in field fall off rate, which has the general effect of making the system quite sensitive to large metallic objects. Also, metallic objects parallel to and near the transmitter loops produce very large eddy current magnitudes which reduces the signal level within the operating volume. In order to reduce the effects of metallic objects near the transmitter in this system, the transmit coils must be placed some distance away from the ground plane in order to reduce signal loss, which occurs when a loop of wire gets close to a conducting ground plane parallel with the plane of the loop. In the case of the planar transmitter configuration in this system, a planar ground plane may be placed some distance below the transmit coils. For zero distance, the magnetic field reduction within the operating volume is nearly total, so one must find a compromise between effective transmitter thickness, defined as the total thickness of the transmit coils, ground plane, and spacing between them, and signal loss. Also, due to the fact that the ground plane eddy current loop area is large with respect to a single transmit coil area, there is an additional degrading effect as the sensor gets further from the transmitter. The ground plane current distribution is similar no matter which transmit coil is operating. This means that the ground plane eddy current field vectors will be similar also. Since the field at any point within the operating volume is the vector sum of transmit coil field minus ground plane eddy field, and the ground plane field effective radius is larger than the transmit coil radius, we can see that the further we get from the plane of the transmitter, the more the field is determined by the ground plane currents. The net effect is that the vectors from the 3 transmit coils are less distinct, which makes the system more sensitive to noise and metallic distortion, as the system uses differences in the vector magnitudes and directions to determine position. As these differences become small, a small change on one of the vectors can result in a large apparent change of receiver position.
U.S. Pat. No. 5,752,513 to Acker, et al. depicts a system which is a subset of the system described by Blood '330, and operation in all respects is identical with respect to non-dipole transmitter properties and metal sensitivity.
U.S. Pat. No. 5,550,091 to Fukuda, et al. depicts a system using a so-called "Helmholtz" arrangement to produce a controlled field within the operating volume. One disadvantage of this system is its bulk, requiring the operating volume to be surrounded by the "Helmholtz" coil assembly. A second disadvantage of this system is that, when placed upon a metallic object such as a steel table, the magnetic field from the transmit coils will be distorted inside of the operating volume.
U.S. Pat. No. 5,640,170 to Anderson discloses a method of positioning a dipole over a specially constructed spiral over a ground plane. The dipole transmitter in this system must be located over the center of the spiral ground plane assembly, which makes patient placement more difficult in a clinical setting, as this placement may interfere with the surgical field during certain procedures. The benefit of this method is that it is possible to locate the transmitter closer to the ground plane, and one does not need to use the "method of images" to solve for position, but the disadvantage of transmitter location over the spiral/ground plane assembly is very similar to the case of a ground plane only.
U.S. Pat. No. 5,198,768 to Keren depicts a surface coil array for use in NMR applications. The system does not determine position, and does not utilize any methods for reducing the effect of nearby metallic objects.
The present invention represents a radical departure from the prior art relating to such transmitting and receiving position and orientation devices insofar as it is capable of satisfying the requirement of insensitivity to metallic objects under and adjacent to the transmitter assembly without exhibiting the disadvantages of signal degradation.